Ten years after the U.S.-led invasion that toppled Saddam Hussein, Sunni, Shi'ite and ethnic Kurdish factions have yet to find a stable power-sharing deal and security is deteriorating.

More than 300 people have been killed in violence over the past week. More than 700 died in April, according to the United Nations, the highest monthly toll in almost five years.

Unidentified gunmen fired on an army checkpoint in Taji, 20 km (12 miles) north of the capital Baghdad overnight, killing four soldiers, police and hospital sources said.

In the western province of Anbar, three soldiers were killed during a two-hour gunbattle with men who attacked a checkpoint controlled by government-backed Sunni fighters in Garma, 9 km (six miles) east of the city of Falluja, police said.

There was no word on casualties among the attackers, or on their identity. Sunni militants in the al Qaeda-linked Islamic State of Iraq have previously targeted security forces and Sunni fighters who cooperate with the Shi'ite-led government.

Recent attacks in Iraq have hit both Sunnis and Shi'ites.

The conflict in neighbouring Syria, complicated by a Sunni-Shi'ite regional proxy war, has further strained Iraq's fragile communal balance and raised fears of a return to the violence of 2006-07, when monthly death tolls sometimes topped 3,000.

Resentful of their treatment by Shi'ite Prime Minister Nuri al-Maliki's government, Iraq's minority Sunnis have been protesting since December with demands that range from repealing laws seen as penalising their minority sect to forming their own autonomous region, akin to that run by the Kurds in the north.

Sunni militants have urged protesters to take up arms against the government.

(Reporting by Kareem Raheem in Baghdad and Kamal Naama in Ramadi; Writing by Isabel Coles; Editing by Alistair Lyon)